1. Technical Field of the Invention
The present invention relates to a light beam scanning optical system, and more particularly to a structure of a light beam scanning optical system incorporated in laser beam printers, facsimiles and the like for scanning a beam receiving surface with a light beam.
2. Description of Related Art
Generally, a light beam scanning optical system is utilized in laser beam printers and facsimiles. A light beam scanning optical system basically comprises a semiconductor laser as a light source, a deflecting means such as a polygon mirror, and an f.theta. lens. The deflecting means is utilized for deflecting a luminous flux emitted from the semiconductor laser at an equiangular velocity, whereby a spot of the luminous flux moves on a light receiving surface in a main scanning direction. With the equiangular velocity, a difference in a scanning speed occurs over a range from a central portion to both edges in the main scanning direction because the light receiving surface is flat. A f.theta. lens is therefore provided for rectifying the difference in the scanning speed.
Since the f.theta. lens must be made by combining various concave lenses convex lenses and, lens planning is extremely complicated. Because of the number of lens surfaces to be ground, manufacturing process of the f.theta. lens requires higher accuracy and higher cost. Moreover, the f.theta. lens has a problem in that various lenses of the f.theta. lens must be made of a limited material which possesses good permeability.
In order to eliminate these disadvantages, in place of the f.theta. lens, the use of an elliptical mirror (Japanese Published Unexamined Patent Application No. 123040/1979), the use of a parabolic mirror (Japanese Patent Publication No. 36127/1980) and the use of a concave reflector (Japanese Published Unexamined Application No., 173212/1986) have heretofore been proposed. However, there are difficulties in manufacturing the elliptical mirror and parabolic mirror with further difficulty in obtaining high manufacturing accurancy.